Lithium silicate glasses or glass ceramics, method for production thereof and use thereof

ABSTRACT

The invention relates to glass ceramics based on the lithium metasilicate system (Li 2 O.SiO 2 (Li 2 SiO 3 )), which are mechanically processible in a simple manner in an intermediate stage of the crystallization and, after complete crystallization, represent a high-strength, highly translucent and chemically stable glass ceramic.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 13/880,894 filed Apr. 22, 2013, now U.S. Pat. No. 9,125,812 B2,which is the U.S. national phase of International Application No.PCT/EP2011/003091, filed Jun. 22, 2011, which claims the benefit ofGerman Patent Application No. 10 2010 050 275.8, filed Nov. 2, 2010, thedisclosures of which are incorporated herein by reference in theirentireties.

BACKGROUND OF THE INVENTION

The invention relates to glass ceramics based on the lithiummetasilicate system (Li₂O.SiO₂ (Li₂SiO₃)), which are mechanicallyprocessible in a simple manner in an intermediate stage of thecrystallization and, after complete crystallization, represent ahigh-strength, highly translucent and chemically stable glass ceramic.

In the lithium oxide-silicon dioxide system, lithium disilicate(Li₂O.2SiO₂ (Li₂Si₂O₅))-glass ceramics are well known from theliterature and several patents are based on this glass ceramic system.For example, in EP-B-536 479, self-glazed lithium disilicate glassceramic objects are thus described for the production of tableware and,in EP-B-536 572, lithium disilicate glass ceramics which can be used byscattering a fine-particle coloured glass onto the surface thereof ascladding elements for building purposes,

A main focus of the patented lithium disilicate glass ceramics residesin dental applications. The lithium disilicate system is very suitablehere for the production of CAD/CAM-processible glass ceramics since thecrystallization is effected here via the lithium metasilicate phase (seeS. D. Stookey: “Chemical Machining of Photosensitive Glass”, Ind. Eng.Chem., 45, 115-118 (1993) and S. D. Stookey: “PhotosensitivelyOpacifiable Glass” U.S. Pat. No. 2,684,911 (1954)). These lithiummetasilicate glass ceramics have such low strengths in this intermediatestage that they can be readily processed by means of CAD/CAM (M.-P.Borom, A. M. Turkalo, R. H. Doremus: “Strength and Microstructure inLithium Disilicate Glass Ceramics”, J. Am. Ceram. Soc., 58, No. 9-10,385-391 (1975) and M.-P. Borom, A. M. Turkalo, R. H. Doremus: “Verfahrenzum Herstellen von Glaskeramiken” (Method for the production of glassceramics) DE-A-24 51 121 (1974)). Only by the subsequent conversion toform lithium disilicate in a second crystallization stage are dentalmaterials with high strengths achieved.

This principle is exploited in order to produce firstly a glass ceramic,in a two-stage crystallization process, which glass ceramic can bereadily processed mechanically, e.g. by means of CAD/CAM processes, andin order to process this subsequently in a second crystallization stageto form dental glass ceramic. This method is suitable in order to beable to use dental restorations according to the so-called chair-sidemethod. In this method, an individually adapted crown/onlay/inlay ismilled out of a glass ceramic block after the first crystallizationstage by means of CAD/CAM, in the dental practice this is subjected tothe second crystallization stage in a special oven and used directly inthe first and only dentist's visit for the patient (DE 10 2005 028 637).An application by the dental technician in the pressing method or inmechanical processing with subsequent characterisation orindividualisation whilst taking into account suitable paints or layerceramics can also be effected.

BRIEF SUMMARY OF THE INVENTION

Starting herefrom, it was the object of the present invention to provideglass ceramics which have improved strength values and also improvedtranslucence and chemical resistance.

This object is achieved by the lithium silicate glasses or glassceramics, the method for producing a dental restoration and shapeddental restoration described herein, and the advantageous developmentsthereof. Uses of the lithium silicate glasses or glass ceramics are alsodescribed.

Within the scope of the present invention, glass compositions weredeveloped in the basic system SiO₂—Li₂O—ZrO₂, which have lithiummetasilicate as only or as main crystal phase (>50%). %). Hereby,zirconia acts as a stabilizer of the residual glassy phase and can becompletely or partially replaced by oxides of Hafnium, Germanium,Cerium, Lanthanum, Yttrium, Titanium and zinc.

Surprisingly, it was shown that lithium metasilicate glass ceramicswhich have excellent strength values, exceptional translucence and verygood chemical resistances can be produced in this system.

It was shown in addition that up to 20% by weight of ZrO₂ or otherstabilizers can be incorporated in the glass without the structure beingsignificantly influenced. Contrary to all expectations, the ZrO₂ orother stabilizers does not hereby crystallise as a separate crystalphase but remains completely or extensively in the amorphous residualglass phase. Because of the high proportion of ZrO₂ or otherstabilizers, the mechanical and chemical resistances are hugely improvedin this amorphous phase, which also leads to improved properties in theentire dental glass ceramic (crystal phase(s) and residual glass phase),such as for example final strength and acid solubility.

The method is also suitable for a two-stage production process from theinitial glass, a partial crystallization of the lithium metasilicatebeing effected in the first processing stage, which enables good CAD/CAMprocessing. In the second processing stage, an increase in the crystalphase proportion (primary lithium metasilicate) is effected, which leadsto the high strength values. The most important cause of thesurprisingly high strengths in the lithium metasilicate system is herebyascribed to the high zirconium oxide or other stabilizers proportion (>8MA).

High translucence is ensured via the low crystallite size in the glassceramics. In addition, good chemical stability is ensured by the highzirconium oxide proportion in the glass phase and the enriched amount ofSiO₂ in the residual glassy phase compared tolithiumdisilicate-glass-ceramics(Lithiumdisilicate=Lithiummetasilicate+SiO₂).

According to the invention, lithium silicate glasses or glass ceramicswith the following composition are provided:

-   -   50 to 75 wt-% SiO₂,    -   10 to 25 wt-% Li₂O,    -   5 to 30 wt-% of a stabilizer selected from the group consisting        of the oxides of Zr, Hf, Ge, La, Y, Ce, Ti, Zn or its mixtures,    -   0 to 8 wt-% K₂O and/or Na₂O,    -   0 to 8 wt-% Al₂O₃, and    -   0 to 15 wt-% additives.

Preferably, the glasses or glass ceramics have the followingcomposition:

-   -   50 to 75 wt-% SiO₂,    -   10 to 25 wt-% Li₂O,    -   5 to 30 wt-% of a stabilizer selected from the group consisting        of ZrO₂ and/or HfO₂,    -   0 to 8 wt-% K₂O and/or Na₂O,    -   0 to 8 wt-% Al₂O₃, and    -   0 to 15 wt-% additives.

More preferably, the glasses or glass ceramics have the followingcomposition:

-   -   50 to 70 wt-% SiO₂,    -   15 to 22 wt-% Li₂O,    -   8 to 20 wt-% of a stabilizer selected from the group consisting        of the oxides of Zr, Hf, Ge, La, Y, Ce, Ti, Zn or its mixtures,    -   0.1 to 4 wt-% K₂O and/or Na₂O,    -   0.1 to 4 wt-% Al₂O₃, and    -   2 to 8 wt-% additives.

In a preferred embodiment, the glasses or glass ceramics have thefollowing composition:

-   -   50 to 70 wt-% SiO₂,    -   15 to 22 wt-% Li₂O,    -   8 to 20 wt-% of a stabilizer selected from the group consisting        of ZrO₂ and/or HfO₂,    -   0.1 to 4 wt-% K₂O and/or Na₂O,    -   0.1 to 4 wt-% Al₂O₃, and    -   2 to 8 wt-% additives.

In a further preferred embodiment, the glasses or glass ceramics havethe following composition:

-   -   50 to 64 wt-% SiO₂,    -   17 to 20 wt-% Li₂O,    -   8 to 20 wt-% of a stabilizer selected from the group consisting        of ZrO₂ and/or HfO₂,    -   1 to 3 wt-% K₂O and/or Na₂O,    -   1 to 3 wt-% Al₂O₃, and    -   4 to 6 wt-% additives.

In a further preferred embodiment the glasses or glass ceramics have thefollowing composition:

-   -   55 to 64% by weight of SiO₂,    -   10 to 20% by weight of Li₂O,    -   8 to 20% by weight of a stabilizer selected from the group        consisting of ZrO₂, HfO₂ or mixtures hereof,    -   0 to 5% by weight of K₂O and/or Na₂O,    -   0.1 to 5% by weight of Al₂O₃ and also 0 to 10% by weight of        additives.

In a further preferred embodiment the glasses or glass ceramics have thefollowing composition:

-   -   55 to 60% by weight of SiO₂,    -   10 to 20% by weight of Li₂O,    -   8 to 20% by weight of a stabilizer selected from the group        consisting of ZrO₂, HfO₂ or mixtures hereof,    -   0 to 5% by weight of K₂O and/or Na₂O,    -   0.1 to 5% by weight of Al₂O₃ and also 0 to 10% by weight of        additives.

Furthermore, a glass or a glass ceramic with the following compositionis preferred:

-   -   55 to 64% by weight of SiO₂,    -   10 to 20% by weight of Li₂O,    -   10 to 20% by weight of a stabilizer selected from the group        consisting of ZrO₂, HfO₂ or mixtures hereof,    -   0 to 5% by weight of K₂O and/or Na₂O,    -   0.1 to 5% by weight of Al₂O₃ and also    -   0 to 10% by weight of additives.    -   A further preferred composition comprises    -   55 to 60% by weight of SiO₂,    -   10 to 20% by weight of Li₂O,    -   10 to 20% by weight of a stabilizer selected from the group        consisting of ZrO₂, HfO₂ or mixtures hereof,    -   0 to 5% by weight of K₂O and/or Na₂O,    -   0.1 to 5% by weight of Al₂O₃ and also    -   0 to 10% by weight of additives.

The stabilizer is preferably ZrO₂ and/or HfO₂. Preferably, thestabilizer is essentially present in an amorphous state.

There may be contained as additives, components selected from the groupconsisting of nucleation agents, fluorescent agents, dyes, in particularglass-colouring oxides, coloured pigments and mixtures thereof, in theglass or in the glass ceramic.

As for all glasses and glass-ceramics, some components have effects onseveral properties. For example, titania can act as nucleation andcolouring agent. Most of the rare earth metal oxides show effects oncolour and fluorescence. Some components can be simultaneouslyamorphous, incorporated in crystalline phases and build own crystallinephases.

The nucleating agents are preferably selected from the group consistingof phosphorous oxide, titanium oxide, tin oxide, mixtures thereof, andnoble metals, preferably in an amount of 1 to 10 wt-%, more preferably 2to 8 wt-% and most preferably 4 to 8 wt-%.

The fluorescent agents are preferably selected from the group consistingof oxides of bismuth, rare earth elements as neodymium, praseodymium,samarium, erbium, and europium, and mixtures thereof, preferably in anamount of 0.1 to 5 wt-%, more preferably 0.5 to 4 wt-% and mostpreferably 1 to 3 wt-%.

The glass colouring oxides are preferably selected from the group ofoxides of iron, titanium, cerium, copper, chromium, cobalt, nickel,manganese, selenium, silver, indium, gold, vanadium, rare earth elementsas neodymium, praseodymium, samarium, europium, terbium, dysprosium,holmium, erbium, yttrium, and mixtures thereof, preferably in an amountof 0.1 to 6 wt-%, more preferably 0.5 to 5 wt-% and most preferably 1 to4 wt-%.

The coloured pigments can be doped spinels, which are comprisedpreferably in an amount of 0.1 to 6 wt-%, more preferably 0.5 to 5 wt-%and most preferably 1 to 4 wt-%.

Further additives are preferably selected from the group consisting ofboron oxide, phosphorus oxide, fluorine, sodium oxide, barium oxide,strontium oxide, magnesium oxide, zinc oxide, calcium oxide, yttriumoxide, titanium oxide, niobium oxide, tantalum oxide, lanthanum oxideand mixtures thereof, which are comprised preferably in an amount of 0.1to 5 wt-%.

According to the invention, a method for the above-described lithiumsilicate glasses or glass ceramics and a method for producing a dentalrestoration comprising the above-described lithium silicate glass orglass ceramic is likewise provided, wherein

-   -   a) an glass is provided as starting material which comprises the        components of the glass ceramic,    -   b) the glass is subjected to a first heat treatment for        producing a glass ceramic which comprises lithium metasilicate        as exclusive or main crystal phase,    -   c) the glass ceramic of b) is subjected to a second heat        treatment, wherein further metasilicate is segregated from the        glass phase. The lithium metasilicate is present as main crystal        phase.

The first heat treatment is thereby effected preferably at a temperatureof 620° C. to 950° C. over a period of time of 1 to 200 minutes. It isparticularly preferred to implement the first heat treatment attemperatures of 650° C. to 750° C. over a period of time of 10 to 60minutes.

The further crystallization of the lithium metasilicate takes placepreferably at temperatures between 800° C. and 1,040° C. over a periodof time of 5 to 200 minutes, particularly preferred between 800° C. and870° C. over a period of time of 5 to 30 minutes.

The lithium silicate glasses or glass ceramics according to theinvention are used as dental material or as component of a dentalmaterial.

According to the invention, a shaped dental product which comprises thepreviously-described lithium silicate glass or the lithium silicateglass ceramic is likewise provided. The shaped dental products arethereby present in particular in the form of an inlay, an onlay, abridge, an abutment, a facing, a veneer, a facet, a crown, a partialcrown, a framework or a coping.

The lithium silicate glasses or glass ceramics with the followingcompositions are further aspects of the present invention:

Composition 1 SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to 30 wt-%Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0 to 15 wt-% Composition 2SiO₂ 50 to 64 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to 30 wt-% Al₂O₃ 0 to 8wt-% K₂O 0 to 8 wt-% additives 0 to 15 wt-% Composition 3 SiO₂ 55 to 60wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to 30 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8wt-% additives 0 to 15 wt-% Composition 4 SiO₂ 50 to 75 wt-% Li₂O 15 to22 wt-% ZrO₂ 5 to 30 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 15 wt-% Composition 5 SiO₂ 50 to 75 wt-% Li₂O 17 to 20 wt-% ZrO₂ 5 to30 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0 to 15 wt-%Composition 6 SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 8 to 20 wt-%Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0 to 15 wt-% Composition 7SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 10 to 15 wt-% Al₂O₃ 0 to 8wt-% K₂O 0 to 8 wt-% additives 0 to 15 wt-% Composition 8 SiO₂ 50 to 75wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to 30 wt-% Al₂O₃ 0.1 to 5 wt-% K₂O 0 to 8wt-% additives 0 to 15 wt-% Composition 9 SiO₂ 50 to 75 wt-% Li₂O 10 to25 wt-% ZrO₂ 5 to 30 wt-% Al₂O₃ 1 to 3 wt-% K₂O 0 to 8 wt-% additives 0to 15 wt-% Composition 10 SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5to 30 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0.1 to 5 wt-% additives 0 to 15 wt-%Composition 11 SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to 30 wt-%Al₂O₃ 0 to 8 wt-% K₂O 1 to 3 wt-% additives 0 to 15 wt-% Composition 12SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to 30 wt-% Al₂O₃ 0 to 8wt-% K₂O 0 to 8 wt-% additives 1 to 10 wt-% Composition 13 SiO₂ 50 to 75wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to 30 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8wt-% additives 2 to 8 wt-% Composition 14 SiO₂ 50 to 75 wt-% Li₂O 10 to25 wt-% ZrO₂ 5 to 30 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 4to 6 wt-% Composition 15 SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to30 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 5 wt-% Composition 16 SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to30 wt-% P₂O₅ 2 to 8 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 7 wt-% Composition 17 SiO₂ 50 to 75 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to30 wt-% P₂O₅ 4 to 6 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 9 wt-% Composition 18 SiO₂ 55 to 64 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to30 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 5 wt-% Composition 19 SiO₂ 55 to 64 wt-% Li₂O 15 to 22 wt-% ZrO₂ 5 to30 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 5 wt-% Composition 20 SiO₂ 55 to 64 wt-% Li₂O 17 to 20 wt-% ZrO₂ 5 to30 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 5 wt-% Composition 21 SiO₂ 55 to 64 wt-% Li₂O 10 to 25 wt-% ZrO₂ 8 to20 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 5 wt-% Composition 22 SiO₂ 55 to 64 wt-% Li₂O 10 to 25 wt-% ZrO₂ 8 to15 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0 to 8 wt-% additives 0to 5 wt-% Composition 23 SiO₂ 55 to 64 wt-% Li₂O 10 to 25 wt-% ZrO₂ 5 to30 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0.1 to 5 wt-% K₂O 0 to 8 wt-% additives0 to 5 wt-% Composition 24 SiO₂ 55 to 64 wt-% Li₂O 10 to 25 wt-% ZrO₂ 8to 20 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 1 to 3 wt-% K₂O 0 to 8 wt-% additives0 to 5 wt-% Composition 25 SiO₂ 55 to 64 wt-% Li₂O 10 to 25 wt-% ZrO₂ 8to 20 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0 to 8 wt-% K₂O 0.1 to 5 wt-%additives 0 to 5 wt-% Composition 26 SiO₂ 55 to 64 wt-% Li₂O 10 to 25wt-% ZrO₂ 8 to 20 wt-% P₂O₅ 1 to 10 wt-% Al₂O₃ 0 to 8 wt-% K₂O 1 to 3wt-% additives 0 to 5 wt-%

BRIEF DESCRIPTION OF THE DRAWINGS

The subject according to the application is intended to be explained inmore detail with reference to the subsequent figures and exampleswithout restricting said subject to these variants.

FIG. 1 is a Scanning Electron microscope (SEM) micrograph of a glassceramic known from the prior art.

FIG. 2 is a Scanning Electron microscope (SEM) micrograph of a glassceramic according to the present invention.

FIG. 3 is a Scanning Electron microscope (SEM) micrograph of a glassceramic with a low content of stabilizer.

DETAILED DESCRIPTION OF THE INVENTION

As can be seen from the figures, the glass ceramic according to thepresent invention shows much better results resulting in a highertranslucency as the prior art glass ceramic of FIG. 1.

The glass ceramic of FIG. 3 has a lower amount of stabilizer (4 wt-%)and shows a number of white spots of the stabilizer (ZrO₂) which resultsopaque ceramic which is undesirable in the dental field.

Example 1

In Table 1, compositions which are given by way of example arementioned, from which high zirconium oxide-containing metasilicate glassceramics can be produced for the dental field.

TABLE 1 (Data in % by weight) G1 G2 G3 G4 G5 G6 SiO₂ 63.5 63.5 59.0 59.063.5 63.5 Li₂O 12.9 13.9 18.0 19.0 12.9 12.9 ZrO₂ 10.0 9.0 12.0 12.012.3 11.0 Al₂O₃ 4.7 5.1 4.5 4.5 3.9 4.4 P₂O₅ 4.5 4.5 3.5 3.5 3.7 4.2 K₂O4.4 4.0 3.0 2.0 3.6 4.0

The glasses were melted at 1,500° C. and poured into metal moulds toform blocks. The blocks were stress-relieved in the oven at 560° C. andcooled down slowly. For the various characterisation processes, theglass blocks were divided up and subjected to a first crystallizationtreatment. For this purpose, the glasses were stored for 10 to 120minutes at 600° C. to 750° C. As a result of this, glass ceramics withstrength values of 150 MPa to 220 MPa (measured according to DIN ISO6872) were produced. Exclusively lithium metasilicate was herebyestablished as crystal phase. In this state, processing by means ofCAD/CAM methods is possible very readily.

With a second short crystallization at 800° C. to 950° C. for 3 to 15minutes, the crystallization is continued and the result is an increasein strength from 300 MPa to 450 MPa (measured according to DIN ISO6872). In addition to the lithium metasilicate phase, a zirconiumoxide-containing subsidiary crystal phase can hereby be produced. Also asmall conversion of lithium metasilicate into lithium disilicate ispossible. The unambiguous main crystal phase remains the lithiummetasilicate.

In Table 2, the crystallization conditions of individual glasses andalso the resulting crystal phases and strength values are displayed.

TABLE 2 Glass G1 G2 G3 G4 G5 G6 1. Crystallization 680° C. 700° C. 690°620° C. 680° C. 700° C. 10 min 40 min 120 min 120 min 20 min 20 min 2.Crystallization 820° C. 850° C. 870° C. 880° C. 830° C. 830° C. 15 min10 min 10 min 8 min 15 min 10 min Crystal phases main phase MetasilicateMetasilicate Metasilicate Metasilicate Metasilicate Metasilicate (>80%)subsidiary — — ZrO₂- ZrO₂- disilicate disilicate phase (<20%) containingcontaining Translucence excellent excellent very good very goodexcellent excellent 3-point bend- 322 MPa 418 MPa 430 MPa 323 MPa 403MPa 402 MPa ing strength

Example 2

In Table 3, fixed compositions given by way of example for differentstabilizer is mentioned, from which high stabilizer-containingmetasilicate glass ceramics can be produced for the dental field.

TABLE 3 in % by weight SiO₂ 60.0 Li₂O 19.0 P₂O₅ 6.0 Al₂O₃ 2.0 K₂O 2.0CeO₂ 1.0 Stabilizer SX* 10.0 *SX represent compositions of thestabilizer S1 to S5 (s. table 4)

Table 4 shows stabilizers used by way of example for dental applicationswith the composition of table 1.

TABLE 4 Stabilizers SX S1 Zirconium oxide: 10% S2 Germanium oxide: 10%S3 Lanthanum oxide: 10% S4 Yttrium oxide: 10% S5 Zirconium oxide: 6%Titanium oxide: 4%

The glasses were melted at 1,500° C. and poured into metal moulds toform blocks. The blocks were stress-relieved in the oven at 560° C. andcooled down slowly. For the various characterisation processes, theglass blocks were divided up and subjected to a first crystallizationtreatment. For this purpose, the glasses were stored for 10 to 120minutes at 600° C. to 750° C. As a result of this, glass ceramics withstrength values of 150 MPa to 220 MPa were produced. Exclusively lithiummetasilicate was hereby established as crystal phase. In this state,processing by means of CAD/CAM methods is possible very readily.

With a second short crystallization at 800° C. to 950° C. for 3 to 15minutes, the crystallization is continued and the result is an increasein strength from 300 MPa to 450 MPa. In addition to the lithiummetasilicate phase, a zirconium oxide-containing subsidiary crystalphase can hereby be produced. Also a small conversion of lithiummetasilicate into lithium disilicate is possible. The unambiguous maincrystal phase remains the lithium metasilicate.

In Table 5, the crystallization conditions of individual glasses andalso the resulting crystal phases and strength values are shown fordifferent stabilizers.

TABLE 5 S1 S2 S3 S4 S5 Crystallization 1 620° C./60 min 540° C./60 min615° C./60 min 620° C./60 min 620° C./60 min Crystallization 2 850° C./8min 820° C./8 min 800° C./8 min 820° C./8 min 820° C./8 min CrystalLi-metasilicate, phases (Li-disilicate, Li-phosphate) Trans- excellentvery good very good excellent Good lucency 3-point- 418 MPa 341 MPa 325MPa 363 MPa 358 MPa bending strength

The invention claimed is:
 1. A lithium silicate glass ceramic having thefollowing composition: 55 to 64 wt-% SiO₂, 10 to 20 wt-% Li₂O, 8 to 20wt-% of a stabilizer selected from the group consisting of ZrO₂, HfO₂,and mixtures of ZrO₂ and HfO₂, wherein said stabilizer is not present asa separate crystal phase but is present in an amorphous residual glassphase, 0 to 5 wt-% K₂O, 0.1 to 5 wt-% Al₂O₃, and 0 to 10 wt-% additives.2. The lithium silicate glass ceramic of claim 1, wherein the stabilizeris a mixture of ZrO₂ and HfO₂.
 3. The lithium silicate glass ceramic ofclaim 1, wherein the additives are selected from the group consisting ofnucleating agents, dyes, glass colouring oxides, coloured pigments, andmixtures thereof.
 4. The lithium silicate glass ceramic of claim 3,wherein the nucleating agents are selected from the group consisting ofphosphorous oxide, titanium oxide, and tin oxide.
 5. The lithiumsilicate glass ceramic of claim 3, wherein the glass colouring oxidesare selected from the group consisting of oxides of iron, titanium,cerium, copper, chromium, cobalt, nickel, manganese, selenium, silver,indium, gold, neodymium, praseodymium, samarium, europium, and mixturesthereof.
 6. The lithium silicate glass ceramic of claim 3, wherein theadditives are selected from the group consisting of boron oxide,fluorine, barium oxide, strontium oxide, magnesium oxide, zinc oxide,calcium oxide, yttrium oxide, titanium oxide, niobium oxide, tantalumoxide, lanthanum oxide, and mixtures thereof.
 7. A method for producinga dental restoration comprising a lithium silicate glass ceramic inaccordance with claim 1, wherein a) a glass is provided as startingmaterial which comprises the components of the glass ceramic, b) theglass is subjected to a first heat treatment for producing a glassceramic which comprises lithium metasilicate as exclusive or maincrystal phase, and c) the glass ceramic of b) is subjected to a secondheat treatment, wherein further metasilicate is segregated from theglass phase and is existent as main crystal phase.
 8. The method ofclaim 7, wherein the first heat treatment is effected with temperaturesfrom 620° C. to 950° C. over a period of 1 to 200 min, and/or the secondheat treatment is effected with temperatures from 800° C. to 1040° C.over a period of 5 to 200 min.
 9. A shaped dental product comprising alithium silicate glass ceramic of claim
 1. 10. The shaped dental productof claim 9, which is an inlay, an onlay, a bridge, an abutment, afacing, a crown, or a partial crown.